by David Wootton
On the night of February 19, 1604, Johannes Kepler was out measuring the position of Mars in the sky with a metal instrument called a quadrant. It was bitterly cold with a biting wind. Kepler found that if he removed his gloves, his hands were soon too numb to manage his instrument; if he kept them on, he could barely make the fine adjustments necessary. The wind was too strong to keep a candle alight, so he had to read his measurements and write them down by the light of a glowing coal. The results, he felt sure, were unsatisfactory – he was out, he thought, by ten minutes of a degree. On a modern school protractor you cannot distinguish ten minutes of a degree, and only one astronomer before Kepler would have thought such a measurement unsatisfactory. The greatest astronomer of the ancient world, Ptolemy, had regarded ten minutes as precisely his acceptable margin of error. But Kepler had worked with Tycho Brahe, who had devised new instruments capable of measuring with unbelievable accuracy, to a single minute.
Kepler was worried about such tiny numbers because he wanted to prove that Tycho’s theoretical tools could not provide an accurate account of Mars’s movement through the heavens – Kepler’s best predictions, using traditional methods, were out by up to eight minutes. By the time Kepler had found a satisfactory way of handling this aberrant eight minutes, he had abandoned the notion that all heavenly movements are circular and introduced the idea of an orbit – the regularly repeated trajectory of an astronomical object through space. This was the culmination of an astronomical revolution that had begun in 1572, with the appearance of a supernova as bright as Venus. According to Aristotle, there was never any change in the heavens, so the nova ought to have been in the upper atmosphere, like a shooting star – but Tycho proved, by measuring parallax (or rather its absence), that it could only be in the heavens. This startling result turned into a large-scale crisis for the old ways of thinking when Tycho’s measurements of the comet of 1577 showed that not only was it in the heavens, but its path cut through the transparent orbs that were supposed to carry the planets – it took Tycho a decade to accept the obvious conclusion that there were no orbs, and that the planets float through space. But not even Tycho could imagine that heavenly movements were anything other than circular.
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